The illness first killed a 60-year-old man exposed to live poultry and then his previously healthy 32-year-old daughter who had been caring for him.

But dozens of other people exposed to the two flu victims did not fall ill, according to a new report published Aug. 6 in the journal BMJ, leading epidemiologists to conclude that the virus's ability to transmit itself between humans is limited at this point.

"To date, it hasn't been sustainable. It's only been one jump, from chickens to humans," said Dr. Marc-Alain Widdowson, who leads the international epidemiology and research team in the U.S. Centers for Disease Control and Prevention's Influenza Division. "It hasn't gone from chicken to human to human to human to human. That's the concern. That's when we have a pandemic on our hands."

The virus, which was first identified in February, has killed one-third of the people who have contracted it. By the end of June, 133 cases had been reported, resulting in 43 deaths.

New cases have slowed to a trickle since May, World Health Organization spokesman Gregory Hartl said. This is likely due to warmer weather in China and the closure of some live bird markets in the affected provinces.

H7N9 has raised concerns among epidemiologists because it can bind to both human and bird cells, unlike the earlier H5N1 bird flu. This increases the possibility that the virus can more easily jump from birds to humans.

"This virus, contrary to H5N1, does seem to have a better affinity to human receptors, so there is a chance that this virus may affect humans more easily," Widdowson said.

The virus attaches itself to cells in the windpipe and lungs, infecting even cells lodged deep in the respiratory system.

A handful of suspected human-to-human transmissions have been noted by epidemiologists since the virus was identified, but this is the first to be published, Hartl said.

The affected father regularly visited a live poultry market and became ill five to six days after his last exposure to poultry, according to the Chinese epidemiologists who wrote the BMJ report.

He was hospitalized on March 11 and transferred to the intensive-care unit on March 15 when his symptoms became worse. He died of multiple organ failure on May 4.

His adult daughter had no known exposure to live poultry but remained by her father's bedside for days after he fell ill, the researchers said. The report noted that she managed his oral hygiene and gave other care without using any personal protective equipment.

She developed symptoms six days after her final contact with her father and was hospitalized on March 24.

Doctors transferred her to the ICU on March 28, and she died of multiple organ failure on April 24.

Researchers found two almost genetically identical virus strains in each patient, suggesting transmission from father to daughter.

Public health officials tracked down 43 close contacts of both the father and daughter, but all tested negative for H7N9 infection.

"This paper is reassuring because many of the other people in contact with the cases did not contract the virus," said Dr. Jeff Duchin, a spokesman for the Infectious Diseases Society of America. "Also, the researchers did not find any genetic changes that would suggest that the virus that did transmit from human to human had adapted or displayed changes that would suggest it would be more likely to cause a human pandemic."

Duchin concluded that the report "doesn't change the threat level."

"This confirms what a lot of people already believed, that there are rare cases of human-to-human transmission," said Duchin, a professor of allergy and infectious disease and an adjunct professor of epidemiology at the University of Washington's School of Public Health, in Seattle.

Nonetheless, the CDC and WHO will continue to monitor the virus's progress, Widdowson and Hartl said.

"It would be wrong for people to let down their guard now. We need to be vigilant, and we need to keep watching for cases," Hartl said. "It's out there still. That we know, because we haven't found the source definitively. So we need to expect to see more cases."

SOURCES: Dr. Marc-Alain Widdowson, lead, international epidemiology and research team, influenza division, U.S. Centers for Disease Control and Prevention; Gregory Hartl, spokesman, World Health Organization; Jeff Duchin, M.D., professor of allergy and infectious disease and adjunct professor of epidemiology, School of Public Health, University of Washington, Seattle, and spokesman, Infectious Diseases Society of America; Aug. 6, 2013, BMJ